Beckhoff EtherCAT – Ethernet for Control Automation Technology
Ultra high speed right up to the I/O
It was presented for the first time at the Hanover exhibition in Germany, and provided a talking point amongst automation
engineers: EtherCAT – the new, real-time Ethernet network from Beckhoff. EtherCAT is remarkable for its exceptional performance, extremely
easy wiring, and its openness to other protocols. Where conventional fieldbus systems come up against their limits, the real-time Ethernet
system is setting new standards.
The properties speak for themselves: 1000 I/O in 30 µs, optionally twisted pair cable
or optical fiber and, thanks to Ethernet and Internet technologies, optimum
vertical integration. EtherCAT gives you the option of using the classic more expensive
star topology or a simple low cost line structure - no expensive infrastructure
components are required. EtherCAT uses very cost-effective standard
Ethernet cards (NIC) while other real-time-Ethernet approaches require special
and expensive cards in the controller.
The EtherCAT operating principle
With EtherCAT technology, Beckhoff overcomes the system limitations of other
Ethernet solutions:The Ethernet packet is no longer received, then interpreted and
copied as process data at every connection. The newly developed FMMU (fieldbus
memory management unit) in each I/O terminal reads the data addressed to
it, while the telegram continues through the device. Similarly, input data are inserted
while the telegram passes through. The telegrams are only delayed by a
Other Ethernet approaches cannot match the EtherCAT real-time capability. One
approach includes disabling the CSMA/CD access procedure via higher level protocol
layers and replacing it with a time slice procedure or a polling procedure.
Another approach uses special switches that distribute Ethernet packets in a precisely
controlled timely manner. These other approaches are all capable, to a certain
degree, of quickly and accurately transferring data from the controller to the
Ethernet node. However, these other approaches are limited because of delays
from the Ethernet node to the actual I/O or drive controllers.The other approaches
require a sub bus especially when using modular I/O systems. The other Ethernet
approaches are made faster through synchronization of the sub bus system much
like Beckhoff has done in the past with other existing fieldbus networks. However,
the synchronization creates small delays to the communication bus that cannot
be avoided. Beckhoff takes the next step in technology using the FMMU technology
Full duplex Ethernet in the ring, one telegram for many devices:
The EtherCAT system architecture increases the “communication efficiency”.
Maximum flexibility for wiring: with or without switch, line or tree topologies can
be freely selected and combined. Cost-effective twisted pair cable, selection of the
transfer physics depending on requirements. Address assignment is automatic;
no IP address setting is required.
Ethernet up to the terminal – complete continuity
The Ethernet backplane for the I/O modules is called E-bus. The E-bus transfers
the data from one I/O point to another using a different electrical signal but not
changing the Ethernet data. The first Ethernet node, called the Bus Coupler, converts
the electrical signal from standard twisted pair or fiber optics to E-bus. The
signal is converted to E-bus to meet electronic terminal block electrical signal re-
quirements. The signal type within the terminal block (E-bus) is also suitable for
transfer via a twisted pair line over short distances (up to 10 m). The terminal
block can thus be extended very cost-efficiently. Subsequent conversion to Ethernet
is possible at any time since the Ethernet data is never changed.
On the control side, very inexpensive, commercially available standard network
interface cards (NIC) are used as hardware in the controller. The cards offered by
Beckhoff bundle up to 4 Ethernet channels on one PCI slot and are based on the
same architecture. The common feature of these interface cards is data transfer
to the PC via DMA (direct memory access), i.e. no CPU capacity is taken up for
the network access.
The NIC cards use the TwinCAT Y driver which operates seamlessly with the software
operating system and the real-time system. This means the TwinCAT Y driver
functions as a compatible network driver, and additionally as a TwinCAT Ethernet
fieldbus card. The real-time system Ethernet frames have priority over the
general operating system frames using an internal prioritization system. The general
operating system’s Ethernet frames, such as print spooling, Internet, and
mail, are transmitted in the “gaps” if sufficient time is available.
At the receiving end, all the Ethernet frames received are examined by the Twin-
CAT I/O system, and those with real-time relevance are filtered out. All other
frames are passed on to the operating system after examination, outside the context
of the real-time system.
Since the Ethernet functionality of the operating system is fully maintained, all
operating system-compatible protocols can be operated in parallel on the same
physical network. This not only includes standard IT protocols such as TCP/IP,
HTTP, FTP or SOAP, but also practically all Industrial Ethernet protocols such as
Modbus TCP, ProfiNet or EthernetIP.
Optimized protocol directly within the Ethernet frame
The EtherCAT protocol uses a special Ether-type inside the Ethernet Frame. The
Ether type allows transport of control data directly within the Ethernet frame
without redefining the standard Ethernet frame. The frame may consist of several
sub-telegrams, each serving a particular memory area of the logical process images
that can be up to 4 gigabytes in size. Addressing of the Ethernet terminals
can be in any order because the data sequence is independent of the physical order.
Broadcast, Multicast and communication between slaves are possible. Transfer
directly in the Ethernet frame is used in cases where EtherCAT components
are operated with TwinCAT and in the same subnet as the control computer.
However, EtherCAT applications are not limited to TwinCAT as the control system:
EtherCAT UDP packs the EtherCAT protocol into UDP/IP datagrams. This enables
any control with Ethernet protocol stack to address EtherCAT systems. Even communication
across routers into other subnets is possible. In this variant, system
performance obviously depends on the real-time characteristics of the control and
its Ethernet protocol implementation. The response times of the EtherCAT network
itself are hardly restricted at all: The UDP datagram only has to be unpacked
in the first station.
Protocol processing completely in hardware: The protocol ASICs are flexibly
configurable. Process interface from 2 bit to 64 kB.
Coupler Terminal I/O E-Bus Terminal Intelligent
physics from Ethernet
Power Supply system:
24 V DC
2 … 4 bit wide I/O data
interface to E-Bus
up to 4 kByte wide
I/O-data and parameter
interface to E-Bus
Power Supply I/O:
5 ... 230 V DC
1000 I/Os in 30 µs | 200 analog I/Os in 50 µs | 100 axes in 100 µs
EtherCAT reaches new dimensions in network performance. Thanks to FMMU in
the terminal and DMA access to the network card in the master, the complete
protocol processing takes place within hardware and is thus independent of the
run-time of protocol stacks, CPU performance or software implementation. The
update time for 1000 I/Os is only 30 µs - including terminal cycle time. Up to 1486
bytes of process data can be exchanged with a single Ethernet frame - this is
equivalent to almost 12000 digital inputs and outputs. The transfer of this data
quantity only takes 300 µs.
The communication with 100 servo axes only takes 100 µs. During this time, all
axes are provided with set values and control data and report their actual position
and status. The distributed clock technique enables the axes to be synchronized
with a deviation of significantly less than 1 microsecond.
The extremely high performance of the EtherCAT technology enables control concepts
that could not be realized with classic fieldbus systems. For example, the
Ethernet system can now not only deal with velocity control, but also with the
current (torque) control of distributed drives. The tremendous bandwidth enables
status information to be transferred with each data item. With EtherCAT, a communication
technology is available that matches the superior computing capacity
of modern Industrial PCs. The bus system is no longer the “bottleneck” of the
control concept. Distributed I/Os are recorded faster than is possible with most
local I/O interfaces. The EtherCAT technology principle is scalable and not bound
to the baud rate of 100 MBaud – extension to GB Ethernet is possible.
Topology – maximum flexibility
Line, tree or star: EtherCAT supports almost any topology. The bus or line structure
known from the fieldbusses thus also becomes available for Ethernet. Particularly
useful for system wiring is the combination of line and branches or stubs:
Power Supply Power Supply
BE1000 ASIC BE1000 ASIC
at the E-Bus
4 … 64 kByte wide
process data and
The required interfaces exist on the couplers; no additional switches are required.
Naturally, the classic switch-based Ethernet star topology can also be used.
Wiring flexibility is further maximized through the choice of different cables. Flexible
and inexpensive standard Ethernet patch cables transfer the signals optionally
in Ethernet mode (100Base-TX) or in E-bus signal representation. Plastic fiber
optics (PFO) can be used in special applications. The complete bandwidth of the
Ethernet network – such as different fiber optic and copper cables – can be used
in combination with switches or media converters.
Fast Ethernet or E-bus can be selected based on distance requirements. The Fast
Ethernet physics enables a cable length of 100 m between devices while the Ebus
line is intended for distances of up to 10 m. The size of the network is almost
unlimited since up to 65535 devices can be connected.
TwinCAT Y driver:
EtherCAT instead of PCI
The central PC becomes smaller and more cost-effective because additional
slots are not needed for interface cards since the onboard Ethernet port can be
used. With increasing miniaturization of the PC components, the physical size of
Industrial PCs is increasingly determined by the number of required slots. The
bandwidth of Fast Ethernet, together with the data width of the EtherCAT communication
hardware (FMMU chip) enables new directions: Interfaces that are
conventionally located in the IPC are transferred to intelligent interface terminals
at the EtherCAT. Apart from the decentralized I/Os, axes and control units,
complex systems such as fieldbus masters, fast serial interfaces, gateways and
other communication interfaces can be addressed. Even further Ethernet devices
without restriction on protocol variants can be connected via decentralized
“hub terminals”. The central IPC becomes smaller and therefore more costeffective,
an Ethernet interface is sufficient for the complete communication
with the periphery.
FMMU (fieldbus memory management unit):
Telegram processing completely in hardware
Precise synchronization through distributed clock
Accurate synchronization is particularly important in cases where widely distributed
processes require simultaneous actions. This may be the case, for example,
in applications where several servo axes carry out coordinated movements simultaneously.
The most powerful approach for synchronization is the accurate alignment of distributed
clocks, as described in the new IEEE 1588 standard. In contrast to fully
synchronous communication, where synchronization quality suffers immediately
in the event of a communication fault, distributed aligned clocks have a high degree
of tolerance from possible fault-related delays within the communication
With EtherCAT, the data exchange is completely hardware based on “mother”
and “daughter” clocks. Each clock can simply and accurately determine the
other clocks’ run-time offset because the communication utilizes a logical and
full-duplex Fast Ethernet physical ring structure. The distributed clocks are adjusted
based on this value, which means that a very precise network-wide timebase
with a jitter of significantly less then 1 microsecond is available.
However, high-resolution distributed clocks are not only used for synchronization,
but can also provide accurate information about the local timing of the data acquisition.
For example, controls frequently calculate velocities from sequentially
measured positions. Particularly with very short sampling times, even a small
temporal jitter in the displacement measurement leads to large step changes in
velocity. With EtherCAT, Beckhoff introduces new, expanded data types (timestamp
data type, oversampling data type). The local time is linked to the measured
value with a resolution of up to 10 ns, which is made possible by the large
bandwidth offered by Ethernet. The accuracy of a velocity calculation then no
longer depends on the jitter of the communication system. It is orders of magnitude
better than that of measuring techniques based on jitter-free communication.
Hot connect and diagnosis
The Hot Connect function enables parts of the network to be linked and decoupled
or reconfigured “on the fly”; offering flexible responses to changing configurations.
Many applications require a change in I/O configuration during operation.
Examples are processing centers with changing, sensor-equipped tool systems
or transfer devices with intelligent, flexible workpiece carriers. The protocol
structure of the EtherCAT system takes account of these requirements.
Special attention was paid to exemplary diagnostic features during the development
of EtherCAT. The Beckhoff comprehensive experience with fieldbus systems
shows that availability and commissioning times crucially depend on the diagnostic
capability. Only faults that are detected quickly and accurately and located
unambiguously can be rectified quickly.
During commissioning, the actual configuration of the I/O terminals is checked for
consistency with the specified configuration. The topology should also match the
configuration. I/O verification is possible during start-up and also via automatic
The process image allocation is freely configurable. Data are copied directly in the I/O
terminal to the desired location within the process image: no additional mapping is required.
Very large address space of 4 GB.
configuration upload because of the built-in topology recognition.
Bit faults during the data transfer are reliably detected through evaluation of the
32 bit CRC checksum, which has a minimum hamming distance of 4. The Ether-
CAT protocol, transfer physics and topology enables quality monitoring of each
individual transmission segment. The automatic evaluation of the associated error
counters enables precise localization of critical network sections. Gradual or
changing sources of error such as EMC influences, defective push-in connectors
or cable damage are detected and located, even if they do not yet overstrain the
self-healing capacity of the network.
| 256 digital I/Os in 12 µs
| 1000 digital I/Os in 30 µs
| 200 analog I/Os (16 bit)
in 50 µs, corresponding to
20 kHz sampling rate
| 100 servo axes in 100 µs
| 12000 digital I/Os in 350 µs
| Throughput: 10 kB/ms, distributed
to 1,500 devices
| Line, tree or star topology
| Up to 65,535 devices
| Network size:
almost unlimited (> 500 km)
| Operation with or without
| Cost-effective cabling: standard
Ethernet patch cable (CAT5)
| Twisted pair physical layer:
| Ethernet 100BASE-TX, up to
100 m between 2 devices
| E-bus, industrial grade Ethernet,
up to 10 m between 2 devices
| Optional fiber optic cable from
50 to 2000 m
| Hot connect/disconnect of
| Network-wide process image:
| Device process image:
2 bit to 64 kB
| Address allocation:
| Device address selection:
automatically via software
The existing wide range of K-bus I/O terminals from the proven Beckhoff Bus Terminal
line can be networked with EtherCAT. The range includes appropriate bus
couplers which are the network interface for the modular I/O terminals. This ensures
compatibility and continuity with the existing system. Existing and future
investments are protected.
| Optimized protocol directly within
the Ethernet frame
| Fully hardware-implemented
| For routing and socket interface:
| Processing while passing
| Distributed clock for accurate
| Time stamp data types for resolution
in the nanosecond range
| Oversampling data types for
| Breaking point detection
| Continuous “quality of line”
measurement enables accurate
localization of transmission faults
Beckhoff has taken every effort to
ensure EtherCAT technology is fully
Ethernet-compatible and truly open.
The protocol tolerates other Ethernet-based
services and protocols on
the same physical network – usually
even with minimum loss of performance.
There is no restriction on
the type of Ethernet device that can
be connected within the EtherCAT
strand via a hub terminal. Devices
with fieldbus interface are integrated
via EtherCAT fieldbus master
terminals. The UDP protocol variant
can be implemented on each socket
interface. Finally, the intention is to
disclose the technology once the
development work is completed.
| Hub terminal for standard
| Fieldbus terminals for fieldbus
| Decentralized serial interfaces
| Communication gateways
| Fully Ethernet-compatible
| Operation with switches and
| Mixed operation with other
protocols also possible
| Internet technologies
(web server, FTP, etc.)
| Compatible with the
existing Bus Terminal range
| Disclosure in preparation